1. Field of the Invention
The present invention relates generally to hard copy apparatus, more particularly to transport methods and devices for moving print media through a hard copy apparatus, and more specifically to a vacuum holddown print media path transport for wet-dye printing which reduces print media cockle.
2. Description of Related Art
Wet-dye hard copy apparatus, such as computer printers, graphics plotters, copiers, and facsimile machines, and the like, must contend with a major problem in that wet-dye saturated print media tends to deform. [For simplification is of discussion, the term "printer" is used hereinafter generically to mean all hard copy apparatus; the term "paper" is used generically hereinafter for all forms of print media. No limitation on the scope of the invention is intended by the inventors, nor should any such limitation be implied.] Wet-dye saturated paper becomes unacceptably wavy, or "cockled," as the dye interacts with the fibers of the paper. Moreover, particularly noticeable in color printing is the tendency of adjacent wet-dye areas to run or bleed into one another.
Commercial ink-jet products such as the Hewlett-Packard.TM. DeskJet.TM. computer printers employ a wet-dye inkjet technology for producing hard copy. The art of inkjet technology is relatively well developed. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994) editions; incorporated herein by reference. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy [sic] Devices, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988). [Note that the term "ink" is used hereinafter also to refer to all liquid wet-dye systems, e.g., whether the apparatus is using ink (where water-based, dye-based or pigment-based), wet toner, or another liquid colorant. No limitation on the scope of the invention is intended by the inventors, nor should any such limitation be implied.]
Typically thermal ink-jet apparatus inks are water-based and when deposited on wood-based papers, they are absorbed into the cellulose fibers, causing the fibers to swell. As the cellulose fibers swell, they generate localized expansions, causing the paper cockle. Not only does this create a finished hard copy product that may be objectionable to the end-user, cockle growth can cause actual degradation of ink dot printing quality itself due to uncontrolled pen-to-paper spacing which may even, in turn, lead to pen printhead-to-paper contact as the cockle waves move a region of the paper upwardly.
Moreover, most commercial ink-jet printers allow the paper to exit the printing zone on a flat platen or into a substantially flat output tray while the ink is drying. A flat platen with no post-printing holddown mechanism allows cockle to expand, generally creating larger waves in the sheet of paper.
Furthermore, in order to produce high quality color copy, e.g., photo-quality printing, ink flux is increased to produce vivid color saturation. This flux increase further exacerbates the paper cockle problem.
Still further, ink-jet printhead size is increasing to increase throughput. As the print zone length increases, ink bleed effects and the paper cockle problem are again enlarged or intensified.
Several solutions to these problems have been developed. U.S. Pat. No. 4,329,295 (Medin et al.) for a Print Zone Heater Screen for Thermal Ink-Jet Printer, U.S. Pat. No.
5,461,408 (Giles et al. ) for a Dual Feed Paper Path for Ink-Jet Printer, U.S. Pat. No. 5,399,039 (Giles et al.) for an Ink-Jet Printer with Precise Print Zone Media Control, U.S. Pat. No. 5,420,621 (Richtsmeier et al.) for a Double Star Wheel for Post-Printing Media Control in Inkjet Printing, and Des. Pat. No. 358,417 (Medin et al.) (each is assigned to the common assignee of the present invention and incorporated herein by reference) exemplify various techniques for a hard copy apparatus using conventional electromechanical paper feed systems.
There remains a need for print zone and post-print zone paper path transport mechanisms that assist in reducing the expanding paper cockle problem. One solution is to hold the paper with a vacuum force. However, another problem has become evident as attempts have been made to employ vacuum forces for holding paper in wet printing environments. For example, with a drum surface employing a field of discrete vacuum holes, the localized vacuum pressure against regions of the underside of the paper adjacent the vacuum holes draws the wet dye through the capillaries of the paper material before the dye has time to set. This results in alternating dark and light concentrations of dye in the final image correlating to the individual vacuum force influence regions of the holes in the field. Again, the non-uniform saturation leads to paper cockle deformation of the paper as the ink dries. It has been found that vacuum holding also reduces the wavelength of the free-growing cockle and creates a higher frequency, or "sharper" looking, cockle wave in the paper.
Therefore, there is a need for vacuum holddown paper path systems that assist in reducing or substantially eliminating paper cockle.